B733; DEPARTING HPN AFTER TWR CLOSED; EXPERIENCED DIFFICULTY CONTACTING N90 DEP DUE TO FREQ CONFUSION. MINOR TFC CONFLICT OCCURRED REQUIRING NO PRECAUTIONARY MANEUVERING.

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CLRNC WAS PICKED UP WHILE ON THE GND AT HPN. UNDERSTANDING TWR WAS CLOSING SOON; TWR ADVISED US TO USE FREQ 120.8 UPON DEP TO CONTACT NY CTL AFTER TWR CLOSED. WX WAS CLR; AND 4 MILES VIS; +10000 FT SCATTERED. LATER; WHILE MONITORING CTAF; JET TFC STARTED UTILIZING RWY 34 FOR DEP/ARRIVALS; DUE TO A WIND SHIFT. TAXI WAS INITIATED WHILE MAKING ADVISORY RPTS ON CTAF; NO TFC WAS NOTED IN AREA; VISUALLY OR VIA RADIO. DURING TAXI; WHILE MONITORING CTAF; CONTACT WAS ATTEMPTED WITH NY ON 120.8. NO CONTACT WAS MADE AFTER NUMEROUS ATTEMPTS. UPON REACHING RWY 34; TWO MORE RPTS WERE GIVEN BY US TO ALERT TFC IN AREA; NO TFC NOTED. WIND 290/6. TRANSPONDER WAS LEFT WITH SQUAWK GIVEN FOR CLRNC TO AIDE IN IDENTIFYING US TO DEPARTURE CTL. NO TFC IDENTIFIED FOR RWY 34/16 OR 11/29. WE DEPARTED VFR. UPON REACHING 1000 FT; TFC WAS VISUALLY IDENTIFIED ON APCH TO RWY 16 AT A CONSIDERABLE DISTANCE AWAY. AT THIS POINT WE WERE ALREADY TURNING LEFT TO A HEADING OF 295 TO FOLLOW SID. NY DEPARTURE WAS CONTACTED AND THEY TURNED US TO A HEADING OF 260. NY QUESTIONED OUR DEP AND WHY THEY WERE NOT CONTACTED ON FREQ 126.4. WE EXPLAINED TWR TOLD US TO USE 120.8 ON DEP. NY CTL THEN SAID; 'AS ILLUSTRATED ON HPN APCH CHARTS; WE WERE TO USE 126.4.' NOWHERE ON ARPT DIAGRAM OR DEP CHARTS WAS 126.4 SHOWN. UPON FURTHER EXAMINATION OF ALL CHARTS; LATER IN FLT; 126.4 WAS FOUND; ON ILS APCH PLATE. BUT APCH PLATE STATES; THAT 126.4 IS TO BE USED ON APCH AND BTWN XA00-XQ00 LOCAL; 120.8 IS TO BE USED BTWN XQ00-XA00; WHICH IS WHAT WE WERE CORRECTLY USING. AT NO POINT WAS THERE A DANGER TO EITHER ACFT AND NO TCAS ALERT OR RESOLUTIONS WERE GIVEN DUE TO THE CONSIDERABLE DISTANCE BTWN BOTH ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR REITERATED HIS UNCERTAINTY AND ATTEMPTS TO ASSURE THE CORRECT FREQ FOR DEP. HE WAS REASSURED BY CLRNC DELIVERY REGARDING THE USE OF THE 120.8 EVEN AFTER THE TWR CLOSED. ATTEMPTS TO DO SO PRIOR TO TKOF WERE UNSUCCESSFUL SO; THE WX BEING VFR; THEY CHOSE TO TKOF AND CONTACT DEP IN THE AIR; A PERFECTLY ACCEPTABLE PROC. THEY ALSO CORRECTLY UTILIZED THEIR ASSIGNED TRANSPONDER CODE TO ASSIST DEP IN IDENTIFYING THEIR ACFT. RPTR FEELS THAT IF THE USE OF A DIFFERENT FREQ AT TIMES OF TWR CLOSURE IS THAT IMPORTANT TO N90 TRACON; THE PROPER IDENTIFICATION OF THAT FREQ SHOULD BE CLEARLY INDICATED AND THAT THE FREQ SHOULD BE THE CONTACT ISSUED IN THE IFR CLRNC.